This is going in AO 2.0.2 (out later today) and will in the future also be added to WP YouTube Lyte and WP DoNotTrack (both of which have a smaller reach).

If you’re a plugin or theme developer and want to warn your users as well (without blocking them), here’s the code I used (do change the translation-domain from “autoptimize” into one that is applicable to your plugin):

<?php if (version_compare(PHP_VERSION, '5.3.0') < 0) { ?>
<div class="notice-error notice">
<?php _e('<strong>You are using a very old version of PHP</strong> (5.2.x or older) which has <a href="http://blog.futtta.be/2016/03/15/why-would-you-still-be-on-php-5-2/" target="_blank"> serious security and performance issues</a>. Please ask your hoster to provide you with an upgrade path to 5.6 or 7.0','autoptimize'); ?>
</div>
<?php } ?>